1. Field of the Invention
The present invention relates to a flowmeter of the type adapted to measure two different physical phenomena or parameters pertaining to a mixed gas and liquid flow in a conduit. From the two measurements obtained, the individual flow rates for the liquid and the gas may be determined by the use of suitable equations. The flowmeter was developed to monitor wet steam streams, although its use is not limited to that application and the flowmeter could readily be used also for the measurement of two phase flows containing two or more components. It could also be adapted for use with gas-slurry flows which may contain more than two phases.
2. Prior Art
The measurement of fluid flow is required in many applications for process control and monitoring.
In two- or multi-phase flow, it is usually desirable to obtain values of the individual phase flowrates.
The accurate metering of the individual phase flow rates in a two phase flow poses difficulties, because neither the density nor the velocity of the fluid remains constant. Thus variations in one parameter are difficult to distinguish from variations in the other parameter.
Among the commonly employed instruments for the measuring of fluid flow are the pitot-static tube meter, the orifice plate meter, and the venturi meter. While these meters are accurate in single phase fluid metering, their accuracy in two-phase flow is limited, because of the aforementioned physical fluctuations. These meters function to create and measure pressure drops due to acceleration or deceleration effects in a closed conduit.
A number of flowmeters are known which monitor two-phase flow by taking two or more independent measurements of different physical parameters of the two-phase flow.
One such flowmeter is disclosed in our earlier U.S. Pat. No. 4,312,234. This meter is operative to create and measure frictional and accelerational pressure drops in a mixture flow. These pressure drops may be correlated with flow rates by the use of theoretical two-phase mathematical models. The device utilizes a twisted tape positioned in the flow to create a frictional pressure drop, and a venturi, for the production of an accelerational pressure drop. Pressure transducers monitor the pressure differentials across the twisted tape and the venturi.
A different approach for the determination of individual gas and liquid flow rates in a two-phase flowstream is exemplified in U.S. Pat. No. 4,178,801, issued to Cassell et al. That patent discloses a centrifugal vapour-liquid separator mounted in the vapour drum of a utility boiler. Such an assembly is principally employed for the physical separation and removal of the liquid phase from the vapour phase, so as to produce substantially liquid-free vapour. It is advantageous in operating such a separator to be apprised for the individual phase flow rates of the two-phase flowstream. Pressure and pressure drop sensors are thus provided in and across the separator. The flowrate of the two-phase flow stream and the flow rates of the individual phases are computed by measuring the pressure differential across the separator, the pressure differential across the flow of the separated vapour phase, and specific absolute pressures. These pressure measurments are correlated with flow rates by the use of mathematical models specific to the separator. This metering technique would, however, not be applicable in the uses contemplated by the flowmeter of the present invention demanding, as it does, the permanent separation of liquid from vapour phase, and two independent flow channels.
Other flowmeters are known which monitor two-phase flow by taking and computing two measurements against theoretical mathematical models. One such meter combines a gamma ray densitometer and a turbine meter. Both of these devices are relatively expensive and not ideal for field use. A second meter proposes the use of a pair of segmental orifices positioned in a conduit. However, the applicability of this meter to general two-phase flow metering is restricted by the limited operating range in which accurate measurements can be made. Both meters have been developed, usually to meet the demands of certain specific applications.
While the two-phase flowmeter disclosed in our '234 patent provided a rugged, inexpensive and accurate meter, the calculations and computations required to arrive at the desired mass flowrates proved exceedingly complex.
Accordingly, there was a need to provide an equally rugged, inexpensive and accurate multi-phase flowmeter which monitors parameters which can more easily be correlated with flow rates, by the use of less complex theoretical mathematical models.